Field of the Invention
The present invention relates to a numerical control device, and more particularly to a numerical control device that can perform automatic inspection of a screw hole with a machine that has machined the screw hole without preparing special dedicated hardware.
Description of the Related Art
Inspection of effective diameters of screw holes (female screws) is generally performed manually with an inspection device called a screw limit gauge. There are two types of gauges, i.e., a Go gauge and a No Go gauge. If a Go gauge is screwed in and passed through the whole length of a screw by hand without difficulty (FIG. 9A) and a No Go gauge is screwed in and not screwed in more than two revolutions by hand without difficulty (FIG. 9B), the screw hole is determined to be acceptable in the inspection. This inspection is defined by JIS standard (JIS B0251).
For automatic inspection of screw holes, there has been a technique of monitoring the torque of a motor having a gauge attached thereto (for example, Japanese Patent Laid-Open No. 2008-261801 and Japanese Patent Laid-Open No. 04-134002). A threshold value is set for the torque. If a Go gauge inserted into a screw hole is passed therethrough without the torque exceeding the threshold value and a No Go gauge reaches the threshold value before the No Go gauge is rotated a predetermined number of revolutions, the screw hole is determined to be acceptable.
Examples of the automatic inspection method also include a noncontact inspection method by an eddy-current flaw detecting device or images (for example, Japanese Patent Publication No. 5582436 and Japanese Patent Laid-Open No. 2007-010620).
As a method for inserting a male screw into a female screw without collapse of a thread, there has been a method in which a male screw that is attached to a motor having a floating mechanism is reversely rotated to match the phase of the thread (for example, Japanese Patent Laid-Open No. 02-224934).
However, conventional techniques have problems described below.
First, an automatic inspection device using a screw limit gauge, which is disclosed in Japanese Patent Laid-Open No. 2008-261801, Japanese Patent Laid-Open No. 04-134002, and the like, is a precision device that requires high accuracy. Accordingly, the automatic inspection device is expensive, thereby causing a problem with respect to cost.
In automatic inspection by torque, only acceptance/defect is determined and a technique for preventing a defect before the defect occurs is not taken into consideration. A main cause of defects of screw holes is the abrasion of taps. When a tap is abraded, a screw hole becomes defective because the screw hole becomes lower than the minimum allowable dimension of the effective diameter or the effective length of the screw part becomes shorter than the allowable dimension. However, only the simple performance of the automatic inspection cannot grasp such a condition.
In many cases, phase matching of a thread is not considered when a gauge is inserted into a screw hole. Thus, a fastening error that is engagement between a gauge and a screw hole may occur. The technique disclosed in Japanese Patent Laid-Open No. 02-224934 or the like may solve this problem but a floating mechanism or the like is required, thereby also causing a problem with respect to cost.
In automatic inspection, the automatic inspection is controlled by executing an inspection program on a numerical control device that controls a said automatic inspection device. However, the inspection program needs to be prepared separately from a machining program. This increases the labor of a worker.